Screws of this type provide a good securement in soft material, in particular also in particle boards, because of the large bearing depth, which results from the large ratio between the outside diameter and core diameter, namely, the threads are very high in relationship to the diameter of the screw. Because of the relatively large pitch (small relationship between outside diameter and axial spacing of the threads) much space remains between the threads, so that the material into which the screw is rotated can engage with relatively large cross sections between the threads. A good cutting action of the threads and a large free space between two adjacent threads is achieved through the small thread angle. However, screws of this type can also be screwed into sheet metal which is not very thick and self-tap grooves therein, into which are received the threads. One can hardly speak of a complete thread in this case.
Screws of this type can be manufactured economically through a chipless shaping. Because of the thin and high thread height, a very large amount of deformation of the screw material is necessary. The forming tools have the tendency to jam up between the steep threads, which creates high friction forces. This can result in untrue or noncircular cross sections of the screws and cause a large amount of wear to the tools.
Furthermore double-threaded screws are known, wherein one screw helix has a higher profile than the helix extending parallel thereto, so that alternately high and low threads appear on the screw.
In both known screws, the height of the thread profile cannot be fully utilized for support purposes because the material which is removed by the threads must find room in the space between the threads and accumulates substantially on the core. Thus a layer is formed on the core, viewed in a simplified manner, which layer prevents the threads from penetrating completely into the material, namely up to the shoulder on the screw core.
The invention provides a screw construction of the above-mentioned type wherein the threads function over their entire height as a supporting surface. The screw is hereby able to be manufactured with less deformation work than the known screws.
This purpose is attained according to the invention by providing a constriction in the area between two threads which project from the core, the most narrow point of which lies approximately in the center between two threads, and the core is tapered starting from the bases of two adjacent threads.
In the inventive screw, the constriction extends helically parallel to the threads. In comparison with the above-mentioned known screw having high and low threads thereon, the inventive screw thus does not have between two threads an elevation in the form of a low thread, but a recess. Material which is removed by the threads can move into this recess. Moving into the recess is favored by the core construction which is tapered starting out from the bases of the thread flanks. In this manner, surfaces are formed on which the material can slide. This makes it possible that the threads penetrate into the material until their base engages the material. As a result, the supporting force is increased. Also the inventive screw can easily be deformed since during the manufacture of the constriction, material is already displaced in direction of the threads, so that the threads can be formed with smaller forces through cold rolling than is possible with a screw having a cylindrical shank. In such known screws having a cylindrical shank very large radial forces are necessary in order to displace the material to the threads. These advantages in the deformation sequence are particularly important in threads having high narrow thread profiles and a large axial pitch, because such screws require, due to their nature, much more deformation than screws having normal threads.
The screw may have a drill point at the tip thereof. It is then suited also for slightly harder materials. It can also be used to drill holes into sheet metal whereby after penetration of the fluted threads the screw is rotated directly into the threaded hole.
The invention can be applied well on screws, which have an outside diameter of between 3 and 6 mm., even though the invention is not to be limited to these sizes.